This invention relates to plasma etch processes used in the manufacture of electronic devices such as liquid crystal displays and integrated circuits. More specifically, the invention relates to processes for etching metal features having a layer of aluminum or aluminum alloy overlying a layer of a refractory metal such as titanium.
Aluminum is the most commonly used material for fabricating electrical conductors, such as contacts and interconnect lines, in thin film transistor liquid crystal displays fabricated on glass substrates and in integrated circuits fabricated on silicon substrates. While such a conductor can be fabricated as a single layer of aluminum or aluminum alloy, an alternative approach is to fabricate the conductor as multiple layers, with a layer of refractory metal below and, optionally, above the aluminum or aluminum alloy layer. The refractory metal advantageously reduces the formation of hillocks in the conductors. In addition, when the conductor is formed over a silicon region to form a contact, a sufficiently thick bottom layer of refractory metal can prevent aluminum atoms from diffusing into the silicon.
Plasma processes for etching aluminum and aluminum alloys typically employ chlorine as the principal reagent for etching the aluminum. Chlorine ions and radicals typically are produced by plasma decomposition of one or more chlorine-containing gases such as Cl2 and BCl3.
One shortcoming of such chlorine-based processes is that they generally etch aluminum faster than they etch the refractory metal. This can cause undercutting of the aluminum side walls of the conductor while the refractory metal below the aluminum is being etched. Undercutting is when a material is excessively etched in a lateral direction so as to produce concave side walls, which is undesirable.
Therefore, a need exists for a plasma process that can etch a layer of aluminum or aluminum alloy overlying a layer of a refractory metal so as to produce a desired straight or tapered side wall contour, without undercutting the side walls.
In one aspect, the invention is a process and apparatus for etching an exposed region of a multi-layer metal having at least two layers: a layer of aluminum or aluminum alloy overlying a layer of refractory metal. The etching process includes at least two steps. In a first step, the aluminum layer is etched by processing the substrate with a first plasma chemistry that etches aluminum. Optionally a portion, but not all, of the lower refractory metal layer also is etched by the first plasma chemistry. In a subsequent second step, the remainder of the refractory metal layer is etched by a second plasma chemistry that etches the lower refractory metal much faster than it etches aluminum.
An advantage of the invention is that it can avoid undercutting the aluminum side wall as the refractory metal layer becomes depleted towards the end of the etch process. Specifically, we discovered that conventional processes for etching an aluminum layer over a refractory metal layer begin to etch the aluminum more isotropically near the end of the etching of the refractory metal, thereby undercutting the aluminum side walls. Our process can reduce or eliminates such undercutting by changing to a second plasma chemistry that etches aluminum either slowly or not at all.
So that the second plasma does not etch aluminum, it preferably does not include boron ions or ions heavier than argon. Preferably, the second plasma includes fluorine-containing species to etch the refractory metal. The fluorine also scavenges any chlorine or other halogens that may have been used during the aluminum etch step, and it can protect the aluminum from further etching by forming a coating of AlF3 that is resistant to etching by chlorine and other reagents.
The same two step process is useful for etching an exposed region of a metal having at least three layers: an upper layer of refractory metal, a middle layer of aluminum or aluminum alloy, and a lower layer of refractory metal. The upper layer and the middle layer are etched with a first plasma chemistry. The process changes to the aforesaid second chemistry (that etches aluminum slowly or not at all) at a point in time after the aluminum layer is removed and before the etch process reaches the bottom of the lower refractory metal layer. In this embodiment, the first plasma chemistry should be capable of etching both the refractory metal of the upper layer and the aluminum of the middle layer.
In another embodiment of the invention, the two step process can be expanded to a three step process for etching an exposed region of a metal having at least the aforesaid three layers. In this embodiment, a first plasma chemistry that does not etch aluminum removes all or most of the exposed upper layer of refractory metal. A second plasma chemistry that etches aluminum (similar to the first step of the two step process) removes the aluminum layer. Optionally, the second plasma chemistry also can etch refractory metal. In a third, final step (similar to the second, final step of the two step process) a third plasma chemistry that etches the lower refractory metal much faster than it etches aluminum removes the lower layer of refractory metal. The time of the transition from the second chemistry to the third chemistry should be after the aluminum layer is removed and before the bottom of the lower refractory metal layer begins to be removed.
As in the two step process, the three step process has the advantage of reducing or eliminating undercutting of the aluminum while the lower refractory metal layer is etched. The three step process has an additional advantage of allowing the aluminum and upper refractory layers to be etched by separate process chemistries that can be optimized for etching aluminum and the refractory metal, respectively. For example, the aluminum can be etched by a process that etches the refractory metal very slowly or not at all.
In any of the above embodiments, the plasma in the final process step preferably further includes oxygen for at least two purposes. One is that the oxygen can scavenge byproducts from the preceding process steps. Another is that the oxygen can remove much of the photoresist overlying the topmost metal layer at the same time the lower refractory metal layer is being etched, thereby reducing the time required for a subsequent photoresist stripping or ashing process.